The present invention relates to the field of electrical power control apparatus, and more specifically to apparatus employing vacuum interrupters.
Vacuum interrupters are widely used in high voltage, high power electrical systems, either as circuit breakers or contactors for large loads.
A vacuum interrupter typically involves a pair of contacts inside a sealed air-tight vessel. One of the contacts is mechanically movable to and away from the other contact, for example by means of a bellows arrangement. By keeping air away from the contacts inside the vacuum interrupter, the interrupter is able to switch large electrical loads without the problem of arcing.
In switching large polyphase loads, generally a plurality of interrupters is provided, one for each phase. a mechanical actuator linkage connects to each of the interrupters and is usually operated electromechanically under control of a device which is a part of a control system. For example, in the case of a polyphase motor, the actuator which mechanically opens and closes the vacuum interrupter on each phase of the supply might be controlled by a motor starting control system, as is generally known in the art.
It sometimes happens that a small leak will develop in the interrupter vessel allowing air to enter. Depending on the amount of air in the vessel, the possibility exists that more severe arcing than is normal may occur within the vessel during an attempt at breaking the circuit. Also, depending on the amount of air in the vessel, this arcing may continue after the contacts are fully opened. In the prior art, the possibility of arcing in a leaky interrupter has generally not been regarded as a serious problem. According to conventional reasoning, since the other interrupters in the remaining phases are assumed good, the other phase circuits will be broken and there will be no possible return path for any arcing current in the leaky interrupter.
It has been discovered, however, that a number of serious problems can result from a leaky vacuum interrupter in a single phase and the present invention has been made to detect such a condition and to activate protection circuity or otherwise warn of the potentially dangerous situation so as to prevent further damage or hazards from developing. One obvious problem which can develop if one interrupter vessel is already leaking, is that if a second interrupter vessel should start leaking, there would be a path for current to flow, resulting in single phase current containing after the supposed opening of the contactor. This could lead to heating and damage to both interrupter vessels, and probably damage to the motor or other load due to operation from a single phase.
A second problem is the possibility of welded contacts. In the first place, it should be recognized that even if an interrupter vessel has leaked full of air, no particular problem is presented during operation of the load, when the contactor is supposed to be closed. The problem results when the contactor is attempted to be opened, which can lead to arcing within the leaking interrupter, through a ground path circuit including surge capacitors and inherent parasitic capacitances. While the ground path current may not be very large compared with the high power levels involved elsewhere in the system, the ground path may be sufficient to establish and maintain arcing through the leaky interrupter during the entire time period that the contactor is supposed to be open. When the contactor is subsequently reclosed, the contacts in the arcing interrupter may have become so hot as to weld together, jamming the entire contactor and preventing it from being released. Alternatively, due to mechanical play in the linkage which actuates all the interrupters, it may be possible that upon reopening, one of the interrupters may actually travel far enough to break its circuit while the welded contact and another remain closed, thus single phasing the load with probable severe damage.
Another potential problem may occur when personnel attempt to open the power cabinet to inspect or work on the contactor. Typically, means are provided so that when the cabinet is opened, high voltage is disconnected from the contactor mechanism by slow moving contacts which are not designed to break load currents. Ordinarily, a mechanical interlock is provided to prevent opening the cabinet unless the actuator is in the off position. However, due to continued arcing or perhaps a welded contact, a considerable amount of current may actually be flowing in one or more of the phases. When the contactor assembly is then disconnected, this current is required to be interrupted by the slow moving contacts. However, these contacts are totally inadequate for handling large magnitudes of current and severe arcing may result, developing into line to line or line to neutral faults.